A kiss is a נשיקה

, so that yesterday’s סבתא

might say to her granddaughter:

תני לי נשיקה!

Give me a kiss!

Now, that’s a kiss. What about to kiss? Depends on the context.

To kiss someone (or something) else is the פיעל verb לנשק

:

כל פעם שהוא נכנס לחדר הוא מנשק את המזוזה.

Every time he comes into the room he kisses the mezuzah. 

To kiss with someone, however, is להתנשק

:

הם ישבו על הספסל והתנשקו.

They sat on the bench and kissed.

The root of these words – נ.ש.ק 

– is probably related to the root נ.ש.כ

meaning to bite: both involve using one’s mouth to make contact with another person or thing. 

להתנשק

, however, is not related to נשק

meaning weapon.